It is known that such a circuit comprises a heat exchanger through which flows the air thereafter introduced in the passenger space, and supplied by the hot engine cooling liquid (generally water). For controlling the heating, its has been proposed to place on the liquid supply and return pipes a cock which can be either in an opened condition in which the hot liquid circulating in the supply pipe flows through the cock, then in the exchanger, and then back through the cock in order to reach the return pipe, or in a closed condition in which the hot liquid circulating in the supply pipe flows in a by-pass channel formed in the cock and back directly in the return pipe, without passing through the exchanger, or again in intermediate conditions in which the hot liquid circulating in the supply pipe flows partly through the cock for supplying the exchanger, and partly through the cock by-pass channel for flowing back directly to the return pipe, thereby providing a progressive control of the vehicle passenger space heating.
In some of such known cocks, the flow of the hot fluid towards the exchanger and/or towards the return pipe via the by-pass channel is controlled by a disc formed with holes and interposed, inside the cock body, between the openings of the pipes into the cock body. Thus, in the closed condition of the cock, a face of said disc is in direct contact with the hot liquid, while its other face is in contact with the liquid which is filling the heat exchanger. The thermal exchange going on through the disc is sufficient for substantially increasing the temperature of the liquid filling the heat exchanger, which is particularly unpleasant for the persons in the vehicle when the weather is hot: the temperature of the air which reaches the inside of the passenger space after having passed through the exchanger which is then, in principle, not supplied, is higher than the outer temperature and heats up the passenger space in an undesirable manner.